There are a variety of chemical vapor deposition (CVD) and etching processes, metallurgical applications and processes for melting high-temperature materials that involve the activation of a gas or gas mixture, and transport of the activated gas to a downstream region in which the plasma is employed to either react and condense to form a film of material on a substrate, react to etch the substrate material and many other uses. The gas may be activated by a number of means such as a hot filament, a microwave discharge, a DC discharge, and plasma jets or torches. In each of these processes, energy is transferred to the gas to create an activated gas containing ions, atoms, radicals, molecules, and/or electrons, many of which may be in excited states.
It is important that the desired activated species remain active until they reach the substrate in the downstream processing area. In microwave reactive gas generators, the gas is typically energized while passing through a microwave waveguide. The substrate to be processed is placed just downstream of the waveguide so that the species are in the useful activated state when they impinge on the substrate. The gas is typically either guided through the waveguide in a tube for activated gas downstream applications, or confined within and activated within a vacuum bell jar placed in the waveguide or a resonant chamber coupled to the waveguide.
In the apparatus employing a gas flow tube passing through the waveguide, the amount of energy that can be coupled to the gas is limited by the flow tube heating caused by heat transfer from the activated gas to the tube. Since the flow velocity is reduced at the tube walls, the gas proximate the walls tends to heat more quickly than the gas passing through the center of the tube, and the plasma discharge may even localize near the walls, thereby overheating the flow tube. Accordingly, such downstream microwave activation devices are able to couple only a relatively small amount of power to the flowing gas, making downstream processing rates slow.
In attempts to overcome this problem, there have been proposed systems requiring the use of metal electrodes in contact with the activation region. However, any electrode in close contact with the active gas leads to impurities and inefficiency.
One example of such a microwave gas activation device is disclosed in U.S. Pat. No. 4,767,608, issued on Aug. 30, 1988, to Matsumoto et al. The Matsumoto et al. patent discloses the generation of a plasma using electric discharges and microwaves, in which the plasma is expanded adiabatically to precipitate diamond-like carbon. The conducting electrodes in the plasma formation region lead to contamination of the plasma and so the end product, as well as decreasing the plasma formation efficiency due to the cooling of the gas by heat transfer to the electrodes.